1. Field of the Invention
The present invention relates to the discovery of synergistic blends of ingredients and a process to enhance peroxide and oxygen and combinations of the two bleaching processes used for the production of paper pulps and textiles. Bleaching activity is enhanced beyond the additive effects of the individual ingredients.
2. Background and Description of the Prior Art
Bleaching of lignocellulosic materials can be divided into lignin retaining and lignin removing bleaching operations. In the case of bleaching high yield pulps like Groundwood, Thermo-Mechanical Pulp and Semi-Chemical pulps, the objective is to brighten the pulp while all pulp components including lignin are retained as much as possible. This kind of bleaching is lignin retaining. Common lignin retaining bleaching agents used in the industry are alkaline hydrogen peroxide and sodium dithionite (hydrosulfite).
Hydrogen peroxide decomposes into oxygen and water with increasing pH, temperature, heavy metal concentrations, etc. The decomposition products, radicals like HO. and HOO., lead to lower yields by oxidation and degradation of lignin and polyoses. Therefore, hydrogen peroxide is stabilized with sodium silicates and chelating agents when mechanical pulps (high yield pulps) are bleached.
The bleaching effect is achieved mainly by the removal of conjugated double bonds (chromophores), by oxidation with hydrogen peroxide (P), or reduction with hydrosulfite (Y). Other bleaching chemicals more rarely used are FAS (Formamidine Sulfinic Acid), Borohydride (NaBH.sub.4), Sulfur dioxide (SO.sub.2), Peracetic acid, and Peroxomonosulfate under strong alkaline conditions.
Pretreatments including electrophilic reagents such as elemental chlorine, chlorine dioxide, sodium chlorite and acid H.sub.2 O.sub.2 increase the bleaching efficiency of hydrogen peroxide bleaching.
In the case of bleaching chemical pulps like kraft pulp, sulfite pulps, NSSC, NSSC-AQ, soda, organosolv, and the like, that is to say with lignocellulosic material that has been subjected to delignifying treatments, bleaching includes further lignin reducing (delignifying) reactions. Bleaching of chemical pulps is performed in one or more subsequent stages. Most common modern bleaching sequences are CEH, CEHD, CEHDED, CEDED, C.sub.D EDED, O.sub.2 C.sub.D EDED, O.sub.2 DE.sub.PO DEP and C.sub.D E.sub.O DE.sub.P D. (C chlorination, E caustic extraction, H alkaline hypochlorite, D chlorine dioxide, O.sub.2 oxygen delignification, C.sub.D chlorination substituted with chlorine dioxide, E.sub.O pressurized extraction with oxygen, E.sub.OP pressurized extraction with oxygen and peroxide, P peroxide, E.sub.P extraction with peroxide.)
In all of these bleaching sequences, the first two stages are generally considered as the "delignification stages". The subsequent stages are called the "final bleaching". This terminology describes the main effects that can be seen by the specific chemical treatments.
While in the first two stages the most apparent effect is the reduction of residual lignin, in the subsequent stages the most distinguishable effect is the increased brightness.
Hydrogen peroxide, oxygen, and combinations of the two bleaching compounds have been used in bleaching paper pulp and textiles for a number of years. Environmental pressure on chlorine based bleaching and the effect it has on effluent from the manufacturing process has accelerated the use of chlorine free bleaching processes to reduce the amount of dioxins and AOX, absorbable organic halides, in the effluent and bleached paper or textiles.
Oxygen bleaching is conducted under alkaline pH conditions at elevated temperature and pressure, with the process generating some peroxide in-situ during the reaction. Peroxide bleaching is also conducted under alkaline pH conditions, normally at elevated temperature. Oxygen stages are being enhanced with the addition of peroxide. There is a trend in caustic extraction stages (wash out lignins) to pressurize the stage and add oxygen or peroxide and sometimes both to enhance bleaching performance. Ozone bleaching is beginning to make an impact. All of these alternative methods are being installed or enhanced in mills to allow the reduction or elimination of the dependency on chlorine based stages.
It is well known that peroxide bleaching compounds, particularly hydrogen peroxide, require stabilization to prevent the rapid breakdown into water and oxygen induced by heavy metals. Iron, copper, and manganese ions, either in process water or bound to the bleachable material, have a catalytic effect on the breakdown of the peroxide, especially at higher alkalinity levels. This results in a loss of peroxide and a lower bleaching efficiency. Chelating agents, such as EDTA (ethylenediamine tetraacetic acid), DTPA (diethylenetriamine pentaacetic acid), gluconic acid, glucoheptanoic acid, tartaric acid, citric acid, polyphosphates, hydroxyalkanephosphonic acid, and aminoalkanephosphonic acids, along with their corresponding alkali metal salts, are well known to prevent the breakdown of peroxide by forming complexes with the metals, rendering them harmless to the peroxide. Chelating agents have been used directly in bleach liquor to stabilize the peroxide. Chelation or Q stages have also been used recently as a low pH washing stage in paper pulp bleaching to remove metals from the pulp prior to peroxide, oxygen, or ozone bleaching.
Magnesium sulfate, magnesium chloride, and magnesium oxide have a stabilizing effect on the perhydroxyl ion formed in alkaline peroxide bleaching. EQU H.sub.2 O.sub.2 +NaOH.fwdarw.Na.sup.+ +OOH.sup.- +H.sub.2 O
Magnesium salts also retard the depolymerization of cellulose, which causes loss in strength, that can occur in oxygen or peroxide bleaching stages. Alkali metal silicates are also used in stabilizing peroxide bleach, but pose a significant risk in the formation of insoluble silicate scale later in the process.
U.S. Pat. No. 4,938,842 discloses a peroxide bleaching process employing magnesium sulfate, sodium silicate and a chelating agent.
U.S. Pat. No. 4,849,053 discloses a peroxide bleaching process in which pulp is pre-treated with stabilizing chemicals including magnesium salts and chelating agents such as EDTA.
U.S. Pat. No. 4,619,663 discloses stabilizing compositions (and process) for peroxide textile bleaches comprising metal chelating agents (such as diethylenetriaminepentaacetic acid) and sodium tetraborate decahydrate.
Many patents disclose the use of chelating agents such as polyaminocarboxylic acids (e.g. DPTA) in combination with other additives, but none were found using sodium gluconate (as stabilizer) therewith. No uses of sodium sulfamate were found in this context.
U.S. Pat. No. 2,927,082 discloses peroxide bleach stabilized with magnesium salt plus gluconic acid, sodium gluconate or the like.
U.S. Pat. No. 4,959,075 discloses silicate- and magnesium-free stabilizer mixtures for stabilizing aqueous peroxide bleaching baths, comprising (A) polyhydroxy and/or hydroxycarboxylic acids and their salts, (B) polyacrylic acids, and (C) polyamine and/or amine polyphosphonic acids.
U.S. Pat. No. 4,740,212 pertains to a process for bleaching cellulosic material with hypochlorous acid in the presence of nitrogen compounds such as sulfamic acid. U.S. Pat. No. 3,801,512 discloses stabilized acidic hydrogen peroxide solutions wherein sulfamic acid is utilized.
There is a continual demand for improved chlorine-free bleaching compositions and processes, particularly those which produce increased brightness in delignified pulps.